1. Field of the Invention
The present invention relates to a humidifier, more particularly, a humidifier applied in a fuel cell system by recycling high-temperature and high-moisture exhaust gas discharged from the fuel cell, and transferring water and heat acquired from the recycling process to air or oxygen about to be drawn into the fuel cell, thus achieving effective humidification and increasing the efficiency of the fuel cell system.
2. Description of the Related Art
The Proton-Exchange Membrane Fuel Cell (PEMFC) consists of a plurality of cell units, with each of the cell unit comprising the bi-polar plate, membrane electrode assembly (MEA) and the gasket, wherein the MEA is formed by applying a layer of catalyst and adhering a layer of carbon cloth or carbon paper on a layer of polymer membrane (such as Nafion manufactured by DuPont); the bipolar plate is formed by conductive material, such as graphite, inlayed with gas passages, which convey gas proportionally and swiftly to the top surface of the MEA, thus generating electronic reactions with the catalyst and electrons and other yields are produced. Such electrons can be formed as utilizable electric current via external bridge passageway, and other yields therefrom (such as water and heat) are to be discharged externally via related means.
Since fuel cells utilize reactions between hydrogen and oxygen/air to generate power, they are considered a clean energy source, for the only waste material discharged during such reaction process is water and heat without any chemical or physical waste being produced that might cause environmental or biological concerns and would require higher costs and complicated processing procedures, as do other types of energy-generating sources.
Volumes of electric current and power generated by fuel cells determine the efficiency of fuel cells, and factors controlling volumes of electric current and power generated by fuel cells then include the design of gas passages within the bipolar plate, effective area of the catalyst, the characteristics of polymer membrane material, the thickness and degree of porosity for the electrode layer, wherein the characteristics of polymer membrane material are to separate positive and negative gas molecules and isolate electrons but allow water molecules and hydrogen ions to permeate, thus acquiring the effect of an electrical bridge. Hydrogen ions need to be brought along by water molecules to permeate through the polymer membrane, and hydrogen ions acquire better permeation as the moisture of the polymer membrane goes higher. Therefore, it is one of the key technologies in fuel cells as to how the moisture of the polymer membrane can be kept for acquiring better conduction efficiency out of ions. Please refer to U.S. Pat. Nos. 5,484,666, 6,190,793 and 6,207,312 for detailed structures of PEMFC, bipolar plates and MEAs thereof.
According to the psychrometric chart, the moisture for fully saturated air increases curvingly as temperature rises. For example, under the constant temperature of 25° C., the partial pressure for saturated water vapor is 0.032 kg/cm2, whereas under the constant temperature of 65° C., the partial pressure for saturated water vapor reaches 0.245 kg/cm2. Since the operational temperature for PEMFCs is between 60° C. to 85° C., air before being introduced into a fuel cell may be fully saturated already, yet as such air is introduced into a fuel cell, the relative moisture for such air is swiftly lowered by heightened temperature, thus such air is caused to have strong moisture absorption capacity after entering a fuel cell. Therefore, as such air enters a fuel cell, such air immediately absorbs the internal moisture of the fuel cell when contacting the polymer membrane therein, thus causing the polymer membrane to contain such a low degree of moisture that not only decreases the conduction capacity of the membrane ions but also decreases the efficiency of the fuel cell.
Therefore, it is crucial for the efficiency of fuel cells to properly heat and moisturize air before such air enters fuel cells. Since the moisture-containing capacity of air increases as temperature rises, sufficient heating of air should correspondently proceed as air is moisturized. Consequently, it is crucial for increasing the efficiency of fuel cells as to how air or oxygen entering fuel cells can be moisturized and heated. In addition, high-temperature and high-moisture air discharged from the negative electrode in the fuel cell or a fuel processor generating hydrogen can be utilized as the best means for pre-heating air/oxygen. Thus it is the most direct and effective way to moisturize and heat the system air by utilizing directly pure water (or water vapor) yielded from the fuel cell (negative electrode). Generally, total heat exchangers are utilized for effective exchange of temperature and moisture, which means the effective exchange of both the sensible heat and the latent heat is acquired simultaneously via total heat exchangers.
The conventional total heat exchangers provide the rotor adsorption and permeable membrane designs, wherein the working principle for the rotor adsorption design is to utilize all kinds of moisture-absorbing material as media for transferring moisture and heat, the total heat exchanging rotor is then caused to rotate through an external generator, thus achieving the exchange of moisture and heat from both the cool and hot airflows, whereas the working principle for the permeable membrane design is to select a material permeable by moisture but not by air or a micro-porous material having excellent moisture-absorbing capacity as the membrane to be placed between a dry and cool airflow and a moist and hot airflow, thus achieving the exchange of water and heat.
However, when applied in fuel cell systems, the total heat exchanger with rotor adsorption design is to cause numerous drawbacks, for example, the selection of adsorbents has to be extremely cautious since adsorbents cause tremendous impact on the function of fuel cells, therefore, once an alkali compound is chosen as the adsorbent, the function of fuel cells is to be adversely affected tremendously with the battery life thereof being significantly limited; furthermore, the cost of utilizing the total heat exchanger with rotor adsorption design is higher and power is needed for generating rotors, thus the electricity expenses for fuel cells become more difficult to be lowered, not to mention the efficiency for the total heat exchanger with rotor adsorption design is generally lower than 50%.
On the other hand, when applied in fuel cell systems, the total heat exchanger with permeation membrane design provides better exchanging effect between water and heat with the cost thereof being lower than that of the total heat exchanger with rotor adsorption design, yet since membrane material is mostly made of polymer or porous material, the heat conductability thereof is to be lower than that of metal material; furthermore, since porous material is utilized as the membrane, when under the conditions of the membrane having large area and the pressure difference being constant on the two sides, gases on both sides having pressure differences are to permeate therethrough, consequently the pores of the membrane are to be clogged either by being deformed through containing moisture or by being accumulated with minute particles in the air, thus causing the system to be unstable and the function thereof to be difficult to control.
Therefore, both conventional humidifying devices applied in fuel cells cause drawbacks requiring improvements.